I am 6'2" 185-195 (depending on time of season) and ride a full suspension cannondale scalpel and was wondering how many psi should I have front and back shocks for trail riding? No big drops or jumps really just a standard type of single track.
It varies by shock/fork and manufacturer. You should consult the tech specs for the specific shock and fork on the manufacturer's website or contact their tech support. I know that's a lame answer, but that is exactly what your mechanic SHOULD do. The actual PSI number is more of the result of the sag measurement, but, the recommended PSI by manufacturer is a great starting point. Your sag is the amount of travel in your shock or fork as a result of you just sitting on the bike in riding gear (no bouncing, or severe movements). For my fox Float RL, it has a recommended air pressure by rider weight, so you would put 95 PSI, if you wanted a plush ride you'd adjust that air pressure for 1 3/8" of sag, if you are looking for an XC set up you'd want 7/8" of sag. Your fork may be way different though, this is why you need to contact your manufacturer. Hope that helps!
I find a good starting point is to set the PSI so you lose about a third of the travel to sag on the front and rear.
Then go and ride...
Did it bottom out? Did it feel to marshmallowy in technical bits? Are you losing too much energy to pedal bob? Then up the PSI. Did it feel harsh and forgiving. Then lower the PSI.
There is no magic answer for you, it depends on how active the suspension is, your weight, the terrain, etc. Only you know when it feels right, and that takes experimentation. Also just about all forks and shocks have at least external rebound and compression adjustment, the same applies to these controls.